I’ve always been a Theresa May fan. When David Cameron became Conservative leader, I pushed for her to be given a senior, substantive role in the shadow cabinet.

In government, we rarely interacted directly, but even our biggest clash showed her in a positive light. My friend and colleague Rohan Silva had developed plans for an ''entrepreneur visa’’ that would make the UK a magnet for start-ups and fast-growing companies. David Cameron, George Osborne and our Lib Dem partners were all for it. There was one problem: Theresa May, then home secretary, was adamantly opposed.

After the normal Whitehall processes had failed, I was deputed to speak to her. It was a bruising encounter. Theresa made a simple argument very directly: the prime minister had given her a clear target, to bring net annual immigration below 100,000. She would not countenance anything that jeopardised that. It was vital to clamp down on all forms of immigration at once. Except, of course, from Europe.

That encounter showed me what a tough and details-focused politician Mrs May is. For the record, we did eventually introduce a (watered-down) entrepreneur visa. But the argument also helped solidify my view that we should leave the EU. We were engaging in deliberate economic self-harm in pursuit of a target that was impossible to meet.

Now, thank goodness, those constraints are gone. We can design policy that works for our economy and society.

Before we debate the precise details of our future immigration system, we need to agree what we want it to do, especially since debate seems polarised between caricatured positions: economic liberals who want to “let everyone in” and nationalists who want to “keep everyone out”.

One argument deployed by pro-immigration policy wonks is the “lump of labour fallacy”. I remember hearing a lot about this when Gordon Brown announced his commitment to “British Jobs for British Workers.”

Gordon Brown, who called for British jobs for British workersCredit:
-/Via FT.com

“Brown has fallen for it ”, the economic liberals chortled. “He doesn’t understand that there isn’t some immutable amount of work to go round. The labour market is dynamic. Immigrants don’t 'take’ jobs that would otherwise be done by British people. They contribute to economic growth, thereby creating more jobs for all.”

Of course that’s partly true. But this benign picture ignores the effect of immigration on specific groups, in particular the low- or unskilled.

This brings us to today’s argument. Those who want to preserve Britain’s open-door-to-Europe posture argue that if we restrict unskilled EU immigrants, the jobs they do – in the hospitality sector, agriculture, NHS etc – would not be taken overnight by idle British workers. They would not be done at all, damaging us for no benefit.

Again, there is some truth. But those making this argument also commit a fallacy: one might call it the “lump of unemployment”. This assumes there is an immutable group too lazy, poorly educated, drug-addicted or socially incapacitated to hold down a job.

There is such an underclass. Shamefully, it has been there for decades. And the pressure to do something serious about it – to eliminate the grotesque generational worklessness, addiction, debt and family breakdown that ought to preoccupy our national conscience – has been relieved by our ability to import cheap labour, to pick up the slack.

Here’s what a successful post-Brexit immigration strategy should look like. First, roll out the red carpet to global stars of business, science, technology and the arts, to enrich everyone.

Second, stop exporting the talent of the future – brilliant students who we educate at our best universities, then kick out so that they build companies abroad.

Third, rip up the dysfunctional visa system and build one incorporating the best technological tools available. Decisions on visas and work permits should be made in hours, not months.

We need a strategy to get the most socially disadvantaged workingCredit:
-/Via Leftfootforward.org

Finally, and most importantly, reduce the numbers of unskilled immigrants while simultaneously increasing the supply of work-ready British citizens. Not kicking out EU citizens already here, who came to Britain lawfully, in good faith; Theresa May needs to reverse her nasty, inhuman position on this.

But in the long term, rather than assume that we’re always going to need foreign workers or things will grind to a halt – shouldn’t it be our number one national priority to make every British citizen employable?

This is, of course, a strategy not a plan. But it’s the vital first step towards an immigration system that works, and is one Britain can be proud of.